Associated with a trend toward miniaturization and high performance of electronic device in recent years, in printed wiring boards, density growth and high integration of the wiring density is developed. Along with this, there is an increased demand for enhancement of reliability by increasing the heat resistance of laminates for wiring. In particular, with regarding to the package substrates for semiconductor, a low coefficient of thermal expansion is demanded, since at the time of component mounting or package assembling, a warpage resulting from a difference in the thermal expansion coefficient between a chip and a substrate is a large problem.
In general, a laminate for printed wiring board is obtained as follows; applying a resin composition which is mainly composed of an epoxy resin onto a glass woven fabric to obtain a prepreg, laminating one or more sheets of this, disposing a copper foil, and heat curing by pressing. The epoxy resin is generally excellent in a balance among insulating properties, heat resistance, costs, and the like. However its coefficient of thermal expansion is large, so that it is attempted to reduce its thermal expansion by choosing an epoxy resin having an aromatic ring or highly filling an inorganic filler such as silica, etc. (see Patent Documents 1 and 2). By filling the inorganic filler in a high proportion, it is also possible to further reduce a coefficient of thermal expansion; however, it is known that an increase of the filling amount of the inorganic filler causes a lowering of insulation reliability resulting from moisture absorption, an insufficient adhesive force between the resin composition layer and the wiring layer, and a failure of press molding. Besides, there is known a method in which an inorganic filler is uniformly dispersed by using a silicone polymer, thereby achieving high filling (see Patent Document 3). However, in an application in a multi-layered wiring board, there was a limit in lowering the coefficient of the thermal expansion by achieving high filling by the inorganic filler.
In addition, it is attempted to realize the low coefficient of thermal expansion by choosing or improving a resin. For example, as known examples of an epoxy resin having an aromatic ring, there is a curable resin composition using an epoxy resin having a naphthalene skeleton (Patent Document 4). Also, conventionally, for lowering the coefficient of thermal expansion of a resin composition for wiring board, as shown in Patent Documents 5 and 6, a method in which a crosslinking density is increased to increase Tg, thereby lowering the coefficient of thermal expansion is general. However, the increase of the crosslinking density shortens a molecular chain between functional groups, and shortening of the molecular chain to a certain extent or more is difficult from the standpoints of reactivity, resin strength, and the like. For this reason, there was a limit in lowering the coefficient of thermal expansion by a method for increasing the crosslinking density.
Also, even in the case of lowering the coefficient of thermal expansion as above mentioned, there was a problem that a warpage was caused due to an internal stress in the manufacturing process at the time of solder mounting, resulting in a connection failure.